1. Field of the Invention
The present invention relates to a developing device as well as to an image forming apparatus and developer conveying method using the device, in particular relating to a developing device using a dual-component developer containing a toner and a magnetic carrier, for use in an image forming apparatus for forming image using the toner based on electrophotography as well as to an image forming apparatus and developer conveying method using this device.
Conventionally, image forming apparatuses based on electrophotography such as copiers, printers, facsimile machines and the like have been known. The image forming apparatus based on electrophotography is constructed so as to form an image by forming an electrostatic latent image on the surface of a photoreceptor drum (toner image bearer), supplying toner to the photoreceptor drum from a developing device to develop the electrostatic latent image, transferring the toner image formed on the photoreceptor drum by development to a sheet of paper or the like, and fixing the toner image onto the sheet by means of a fixing device.
Recently, in the image forming apparatuses capable of reproducing full-color and high-quality images, a dual-component developer (which will be referred to hereinbelow as simply “developer”), which can present an excellent stability of charge on the toner, is often used.
This developer consists of a toner and a carrier, which are agitated in the developing device and frictionally rubbed with each other, thereby producing a suitably electrified toner.
In the developing device, the electrified toner is supplied to a developer supporting member, e.g., the surface of a developing roller. The toner thus supplied to the developing roller is moved by electrostatic attraction to the electrostatic latent image formed on the photoreceptor drum. Hereby, a toner image based on the electrostatic latent image is formed on the photoreceptor drum.
2. Description of the Related Art
Besides, recently there is a demand for high-speed performance and miniaturization on image forming apparatuses, it is hence necessary to electrify the developer quickly and sufficiently and also convey the developer rapidly.
Accordingly, a circulating type developing device has been adopted in some image forming apparatuses in order to promptly diffuse supplied toner into the developer and produce an appropriate amount of static charge on the toner. The circulating type developing device includes a developer conveying passage through which the developer is circulatively conveyed, and a developer conveying member which agitates and conveys the developer through the developer conveying passage (JP10-063081A).
The above-mentioned circulating type developing device is constructed such that a toner is supplied from a toner hopper to the developer conveying passage when the toner concentration in the developer inside the developing device becomes lower than a predetermined level, and the supplied toner and the developer are conveyed whilst being agitated.
However, in the above-described conventional circulating type developing device, the supplied toner is conveyed while being mixed up (agitated) with the developer. If the agitating force is low, there occurs the problem that the toner is conveyed to the developing roller without gaining a sufficient amount of charge. On the other hand, if the agitation performance is enhanced by providing many parts such as agitating paddles, there occurs the problem that the developer receives much stress and hence is lowered in durability.
Particularly, if the agitating force is enhanced with respect to a dual-component developer including a micro-sized carrier and a micro-sized toner, the fluidity enhancer particles (external additive) for the toner gets stuck to the surface of the toner due to stress. Hence the fluidity of the developer lowers extremely. Accordingly, a defective agitation, so-called Debemoko phenomenon, which makes it difficult to convey the developer, becomes prone to occur. As a result, the necessary amount of toner cannot be supplied to the surface of photoreceptor drum, causing the problem of lowering image density.